1. Field of the Invention
This invention relates to stitching systems utilizing a bobbin case assembly from which a stored supply of thread is drawn and, more particularly, to a bobbin case assembly having an associated thread tensioning assembly which produces a controlled resistance to payout of thread from a supply thereof.
2. Background Art
In sewing/stitching operations, and particularly in embroidery operations, the tension of two source components forming the lockstitch needle thread and bobbin thread must balance to achieve a high quality stitch. If the tension in the needle thread is significantly greater than the bobbin thread tension, the bobbin thread can be pulled through from the underside of the fabric and show at the top side of the fabric being sewn. This condition can cause puckering of the fabric or disfigured sewing to occur. If the needle thread tension is significantly less than the bobbin thread tension, loops can form on either side of the fabric and the stitching formation can appear loose or distortedly large.
A primary job of a sewing equipment operator is to keep bobbin and needle thread tensions as close as possible to balanced. This method of balancing thread tension has historically been carried out by having the sewing equipment operator observe the pattern after stitches are laid down. Good sewing equipment operators constantly adjust the tension of both needle and bobbin threads to maintain a proper balance. Less skilled operators may not consistently maintain this balance as a result of which poor quality stitching formation may result.
In FIGS. 1 and 2 herein, a conventional sewing/stitching system is shown at 10. The sewing/stitching system 10 consists of a bobbin case assembly at 12 that is operably mounted upon a support 14. The bobbin case assembly 12 consists of a bobbin basket assembly 16, which has a bottom wall 18 and an annular, peripheral wall 20 extending upwardly therefrom, and defining in conjunction therewith, a receptacle 22 for a bobbin 24. The bobbin 24 consists of a cylindrical core 26 having a central axis 28. Disk-shaped flanges 30, 32 are connected to the core 26 at the axial ends thereof, and define in conjunction therewith, a thread storage space 33. A supply of thread 34 is wrapped around the core 26 between the flanges 30, 32. A mounting post 36 projects upwardly from the bottom wall 18 and extends through the bobbin 24 so as to support the bobbin 24 for rotation around the axis 28. The mounting post 36 extends fully through the bobbin 24 and has an exposed portion 38, to which a bobbin case 40 is releasably connected.
The bobbin case 40 and bobbin basket assembly 16 cooperatively define a wall structure 42 that captively maintains the bobbin 24 in an operative position relative to the support 14. Through this arrangement, the bobbin 24 is allowed to rotate relative to the wall structure 42 and support 14 around the axis 28.
The thread 34 is directed through the wall structure 42 to be engaged by a thread drawing mechanism 44. The drawn thread is manipulated by one or more stitching components 46 through which the thread 34 is sewn or stitched in any conventional manner with which those skilled in the art are familiar.
The bobbin case 40 has a peripheral wall 47 which surrounds the peripheral wall 20 on the bobbin basket assembly 16. The thread 34 may extend from the supply through one or both of the peripheral walls 20, 47 to be engaged by the thread drawing mechanism 44. In this embodiment, the peripheral wall 47 has a thread receiving opening 50 formed therethrough. A slot 52 extends through the peripheral wall 47 from one axial end thereof in an L shape up to the thread receiving opening 50.
A thread tensioning assembly at 54 is provided on the peripheral wall 47 and is in the form of a spring element 60 that is curved to nominally match the curvature of the outside surface 62 of the peripheral wall 47. The spring element 60 is maintained on the peripheral wall 47 by a screw fastener 64. The curved spring element 60 overlies all, or part, of the thread receiving opening 50, and a portion of the slot 52. The spring element 60 has an elongate body 66 with a mounting portion 68 that is fixed to the peripheral wall 47 through the screw fastener 64. The free end 70 of the body 66, remote from the mounting portion 68, has an offset finger 72 which projects into an opening 74, and interacts with an edge 78, bounding the opening 74, in such a manner that the free end 70 is confined against axial shifting relative to the peripheral wall 47. A second offset finger 80 on the spring element 60 projects into a slot 82 in the peripheral wall 47, likewise to consistently locate the spring element 60 by preventing axial shifting thereof relative to the peripheral wall 47.
Thread 34 departing from the supply on the bobbin 24 and projecting through the thread receiving opening 50, resides between the spring element 60 and the outside surface 62 of the peripheral wall 47. A captive frictional force can be generated on the thread 34 between the radial inwardly facing surface 84 on the spring element 60 and the outside surface 62 of the peripheral wall 47. The captive pressure applied on the thread 34 can be varied by repositioning a flexing portion 86 of the body 66 relative to the mounting portion 68 of the body 66 through an adjustment screw 88. The user sets the adjustment screw 88 to select a desired frictional resistance force between the thread 34 and surfaces 62, 84 to set a “draw tension” for the bobbin case assembly 12.
Typically, the spring element body 66 is made from a thin piece of spring metal which has a uniform thickness. By turning the adjustment screw 88, the inwardly facing surface 84 on the spring element 60 is selectively moved towards the outside surface 62 of the peripheral wall 47 and allowed to move away therefrom, whereby the frictional resistance force on the thread 34 is varied. The amount of friction, and thus the resulting draw tension, is generally determined on a trial-and-error basis. That is, the user roughly sets the adjustment screw 88 to set a thread draw tension, estimated to be at least within a reasonable range of a desired thread draw tension, and then pulls on the thread 34 while holding the bobbin case assembly 12, or thrusts the bobbin case assembly 12 while holding the thread 34. By these procedures, the user can roughly ascertain whether the desired draw tension has been set to within that reasonable range of the desired thread draw tension. Appropriate fine adjustment can thereafter be attempted through manipulation of the adjustment screw 88, with a repetition of the same trial-and-error procedure.
Given the nature of the spring element 60, and its uniform thickness, the bending characteristics of the flexing portion 86 are substantially the same over the entire length of the flexing portion 86. The nature of the spring element 60 and the adjusting structure, i.e. the adjustment screw 88, are such that generally only a relatively gross adjustment in draw tension can be set by the system operator. The spring element 60 has been conventionally made with a construction that is sufficiently stiff that it does not lend itself to fine adjustments that would allow selection of very specific draw tensions that may be desirable for a balanced system capable of producing high quality stitching.
Regardless of skill level, a system operator will generally be incapable of initially setting a desired draw tension or tensions with any predictability. The system operator, by turning the adjustment screw 88, is capable of changing the state of the spring element 60 from one wherein virtually no frictional force is generated upon the departing thread 34, and one wherein the thread 34 becomes locked between the surfaces 62, 84. All settings in between, made through trial and error, may be difficult to select given that a relatively small change in position of the adjustment screw 88 may produce a relatively large change in the thread draw tension. As such, the system operator is relegated to using potentially time consuming and frustrating trial-and-error techniques in attempting to set all draw tensions within the permitted range.
The assignee herein devised an alternative tensioning system, which is the subject of U.S. Pat. No. 6,152,057, which is incorporated herein by reference. In U.S. Pat. No. 6,152,057, an elongate tensioning element is incorporated and has a cylindrical surface against which thread bears to produce a frictional force. By varying the contact area between the thread and tensioning element, different draw tensions can be set for the system. The system lends itself to wrapping of the thread around the tensioning element, with the degree of wrapping dictating the frictional resistance force between the thread and tensioning element. The structure disclosed in U.S. Pat. No. 6,152,057 does offer significant advantages compared to the prior art system described above with respect to FIGS. 1 and 2 herein.
In high volume sewing operations, there may be a large number of bobbin case assemblies which require setup on a one-by-one basis and periodic adjustments as these systems are operated. Thus, minimizing adjusting time and simplifying adjustment procedures are key to economical operation of such sewing operations. The industry continues to seek out ways to predictably select draw tensions, at or close to desired values, without complicated setup procedures or excessive adjustment as the system is monitored both at startup and during use.